Whirling Disease

Two fish with dark, bent tails in shallow water
These rainbow trout show the characteristic black tail and skeletal deformities indicative of whirling disease.

©Stephen Atkinson/Oregon State University


Whirling disease is caused by a microscopic parasite from Europe (Myxobolus cerebralis) that can infect some trout and salmon; it does not infect humans. It has been detected in 25 states. During the parasite’s life cycle, it takes on two different forms as spores and requires two hosts: a common aquatic worm (Tubifex tubifex) and a susceptible fish. Cutthroat trout are susceptible, especially during the first months of life. The parasite feeds on the fish’s cartilage, and the infection can cause skeletal deformities, a blackened tail, and whirling swimming behavior. Because the fish cannot feed normally and is more vulnerable to predation, whirling disease can be fatal. No practical treatment exists for fish infected with this disease or for the waters containing infected fish.

Presence in Yellowstone

Whirling disease was first detected in Yellowstone in 1998 in cutthroat trout from Yellowstone Lake. It has since been found in the Firehole, Madison, Gibbon, Gallatin, and Lamar rivers and throughout the Yellowstone Lake watershed. In the lake, the infection has spread to about 20% of the cutthroat trout. The parasite is most prevalent in the two known infected tributaries, Pelican Creek and the Yellowstone River downstream of the lake outlet. Infection has been most severe in Pelican Creek, which once supported nearly 30,000 upstream-migrating cutthroat trout. Significant declines in Pelican Creek’s spawning population have been attributed to the combination of whirling disease and predation by nonnative lake trout in Yellowstone Lake. The finding of adult fish in the lake with the parasite’s spores that survived their initial infection suggests some resilience of Yellowstone cutthroat trout to whirling disease.

Studying the Disease

Yellowstone National Park’s cutthroat trout spawning streams, which vary widely in thermal, hydrological, and geological characteristics, provide an exceptional opportunity to study whirling disease in native trout. Park staff have been working with Montana State University’s Department of Ecology to measure how the infection rate might vary in different stream conditions. Certain fish-eating birds have also been shown to disperse the parasite. Research has shown that the parasite can pass through the gastrointestinal tract of some birds, such as great blue herons, and remain alive.

Results of a 2018 survey suggest that whirling disease risk remains very high in Pelican Creek. Overall, however, it does not appear that whirling disease has spread widely throughout spawning tributaries to Yellowstone Lake. In addition, prevalence of infection in juveniles and adults within the lake remains low. Despite this, there are still many unknowns concerning the parasite, particularly in the unique environmental context of Yellowstone.

Park staff emphasize prevention by educating people participating in water-related activities—including anglers, boaters, or swimmers—to take steps to help prevent the spread of the disease. This includes thoroughly cleaning mud and aquatic vegetation from all equipment and inspecting footwear before moving to another drainage. Anglers should not transport fish or water between drainages and should clean fish in the body of water where they were caught.



Bartholomew, J.L. and P.W. Reno. 2002. The history and dissemination of whirling disease. In J.L. Bartholomew and J. C. Wilson, ed., Whirling disease: Reviews and current topics. Vol. Symposium 29. Bethesda, MD: American Fisheries Society.

Kerans, B.L. and A.V. Zale. 2002. The ecology of Myxobolus cerebralis. In J.L. Bartholomew and J.C. Wilson, ed., Whirling disease: Reviews and current topics, 145–166. Vol. Symposium 29. Bethesda, MD: American Fisheries Society.

Koel, T.M., D.L. Mahony, K.L. Kinnan, C. Rasmussen, C.J. Hudson, S. Murcia, and B.L. Kerans. 2007. Whirling disease and native cutthroat trout of the Yellowstone Lake ecosystem. Yellowstone Science 15(2).

MacConnell, E. et al. 1997. Susceptibility of grayling, rainbow, and cutthroat trout to whirling disease by natural exposure to Myxobolus cerebralis. Whirling Disease Symposium, Logan, UT.

Murcia, S., B.L. Kerans, E. MacConnell, and T.M. Koel. 2006. Myxobolus cerebralis infection patterns in Yellowstone cutthroat trout after natural exposure. Diseases of Aquatic Organisms 71(3):191–199.

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Last updated: May 17, 2023

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